Trust Investments (Proceeds From The Sales Of And The Net Realized Gains On Securities in the NDT and Rabbi Trusts) (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Schedule of Trust Investments [Line Items]
Net Gains (Losses) on Trust Investments		$ (22)	$ 28
Rabbi Trust [Member]
Schedule of Trust Investments [Line Items]
Proceeds from Sales	[1]	25	51
Gross Realized Gains		2	15
Gross Realized Losses		(2)	(3)
Net Realized Gains (Losses)	[2]	0	12
Unrealized Gain (Loss) on Securities	[3]	0
Other than Temporary Impairment Losses, Investments		0	0
Net Gains (Losses) on Trust Investments		0	12
Power [Member]
Schedule of Trust Investments [Line Items]
Net Gains (Losses) on Trust Investments		(22)	19
Power [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member]
Schedule of Trust Investments [Line Items]
Proceeds from Sales	[4]	372	247
Gross Realized Gains		24	21
Gross Realized Losses		(12)	(4)
Net Realized Gains (Losses)	[5]	12	17
Unrealized Gain (Loss) on Securities	[6]	(34)
Other than Temporary Impairment Losses, Investments		0	(1)
Net Gains (Losses) on Trust Investments		$ (22)	$ 16

[1] Includes activity in accounts related to the liquidation of funds being transitioned to new managers.

[2] The cost of these securities was determined on the basis of specific identification.

[3] Effective January 1, 2018, unrealized gains (losses) on equity securities are recorded in Net Income instead of Other Comprehensive Income (Loss).

[4] Includes activity in accounts related to the liquidation of funds being transitioned to new managers.

[5] The cost of these securities was determined on the basis of specific identification.

[6] Effective January 1, 2018, unrealized gains (losses) on equity securities are recorded in Net Income instead of Other Comprehensive Income (Loss).